Toxoplasma gondii establishes an important chronic infection capable of causing life-threatening disease in immunocompromised hosts. The basis of persistence of the infection is the tissue cyst, which is preferentially formed in the brain and remains largely quiescent for the life of the host, but can reactivate and cause disease. To understand the mechanisms of resistance against T. gondii, it is essential to analyze the immune responses to the tissue cyst. However, the information on in this regard is quite limited. To fill this gap, we recently developed a new murine model to examine the activity of immune cells against T. gondii cysts and found that immune T cells most likely have the ability to remove the cysts from the brain of infected mice. These results suggest that it might be possible to develop a vaccine to eliminate the tissue cysts from chronically infected patients. Thus, in this proposal, the specific aims are designed to address three main questions to obtain fundamental information essential for beginning to understand the mechanisms of the T cell-mediated elimination of T. gondii cysts. The first specific aim is to determine whether CD4+ T cells, CD8+ T cells, or both subsets, are required for the elimination of T. gondii cysts from the brain. We will utilize the novel technique of in vivo bioluminescence imaging: specifically, we will determine which subset(s) of adoptively transferred T cells are required for removal of luciferase-expressing T. gondii cysts from the brains of T cell-deficient mice. In the second specific aim, we will define the T cell mechanisms that mediate the elimination of T. gondii cysts from the brain. We will transfer immune T cells from mutant mice deficient in IFN-3, perforin or Fas- ligand into T cell-deficient mice infected with luciferase-expressing parasites and monitor changes in cyst burden in the recipients by in vivo bioluminescence imaging. The third specific aim is to determine if the genotype of T. gondii affects the T cell-mediated removal of cysts from the brain. T. gondii has three predominant genotypes (I, II and III), each of which can infect humans. Thus, it is important, especially from clinical aspects, to determine if T cells recognize and eliminate cysts in a genotype-specific manner. To address this point, we will examine whether a transfer of immune T cells obtained from mice infected with a type II parasite eliminates type III cysts from the brain. The studies in these three specific aims will provide groundbreaking information on the targeting of T. gondii cysts by the immune system and dramatically improve our understanding of resistance to the parasite. This information will also be crucial for developing a novel vaccine to eliminate cysts from patients who have already been infected and to prevent establishment of chronic infection with T. gondii after a newly acquired infection.